Ruiz Vargas, Albert, 2019 New methods for exploring gastrointestinal disorders and diseases based on electrical bioimpedance and optical fibre sensing, Flinders University, College of Science and Engineering

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Abstract

Gastrointestinal disorders and diseases are classified as either, functional, structural or motile. Around one third of the population in Australia report symptoms related with functional gastrointestinal disorders at some point during their lives. Approximately, 50 to 60 million US citizens were affected by gastrointestinal symptoms in 2010.

Electrical bioimpedance is used as a clinical tool in oesophageal studies, but it is still a research tool on other parts of the gastrointestinal tract, especially in the lower gastrointestinal tract. That is why, I have focused on the use of electrical bioimpedance in the lower gastrointestinal tract.

In this thesis, I have developed a bioimpedance measurement system, three impedance measuring probes and multiple custom-made software packages to acquire, save and display data in real-time. The three devices, are as follows: 1) a four-electrode catheter, 2) a novel High-Resolution-Impedance-Manometry (HRIM) catheter with fibre optic pressure sensors and closely spaced impedance electrodes, and 3) a novel four-electrode-contact electrical impedance probe with fibre optic contact pressure and temperature sensors built in it. Furthermore, a 3D model of the gastrointestinal tract was implemented that has provided greater understanding of the experimental results obtained during this thesis.

Different in-vitro and in-silico experiments were performed to investigate: 1) the use of different measuring frequencies and parameters (such as impedance magnitude and phase angle) for bolus transit studies, 2) the differentiation between different luminal contents, and 3) the effect of closer electrode separation on HRIM catheters. In addition, an in-vitro animal and ex-vivo human experiments were performed to investigate the use of monitoring the contact pressure on, and standardisation of bioimpedance measurements.

Results showed that the phase angle had much better correlation with changes in luminal diameter than impedance magnitude. The best frequency to use for differentiating between content (liquid and mixed gas) depended on which impedance parameter was selected, with larger differences observed at 177.81 kHz for phase angle, and 5.62 kHz for impedance magnitude. The custom HRIM catheter had twice the spatial resolution of impedance electrodes compared to commercially available HRIM catheters. As a result, subtle muscular contractions in the colon were identified for the first time in in-vitro animal models.

The results with the pencil probe corroborated the previously reported contact pressure dependency on bioimpedance measurements; which demonstrates the importance of controlling and monitoring the contact pressure. A new method to standardize bioimpedance measurement is suggested. This method provides the resistivity normalised to a contact pressure of 0 kPa and also the ratio of resistivity over contact pressure. Two distinct regions of this ratio were observed during the trials, which might be indicative of damage.

The findings and improvements presented in this thesis will help clinical researchers to gain insight on how the gastrointestinal tract works and to standardize bioimpedance measurements for tissue characterization in the gastrointestinal tract in the following ways. Firstly, it was found that the phase angle parameter can add extra information to lower gastrointestinal bolus transit studies by providing information on bolus type. Secondly, the increased spatial resolution achieved in the HRIM catheter allows initiating accurate bolus transit studies in in-vivo animals, in which the use of an external camera will no longer be possible. Finally, monitoring the contact pressure on bioimpedance measurements will improve the consistency of measurements and allow more quantifiable results.